Fold-thrust belts are common manifestations of crustal shortening at convergent and collisional plate boundaries. They contain a variety of geologic structures that develop over a wide range of length scales. This dissertation focuses on two specific aspects of fold-thrust belts: the veins that develop within them and the evolution of the backstop. Veins are mineral aggregates that precipitate in fractures and a backstop is the rigid body that defines the hinterland-most extent of the fold-thrust belt.
Analysis of veins can provide significant information about the mechanisms of deformation in a fold-thrust belt including paleostress orientations, fracture mechanics, and fluid sources. In this study, veins in the latest Silurian through early Middle Devonian limestones, argillaceous limestones, and shales of the Hudson Valley Fold-Thrust Belt (HVB) were examined in both the field and the lab. Field work included classifying veins based on their context in outcrop, measuring vein orientations for stereographic analysis, and collecting samples for thin section analysis. In addition, petrographic, cathodoluminescence, and environmental scanning electron microscope analyses of oriented thin sections were used to analyze the microscopic scale textures and cross-cutting relationships between veins.
Veins were classified by the structure that they were associated with. The classes recognized include: joint-parallel veins, clay-seam-parallel veins, veins in outer-arc tension gashes in folded beds, veins at a high-angle to flexural-slip surfaces, veins associated with dilation in fold hinge zones, fault-parallel veins, and veins associated with incipient shear. Four major aspects of the veins were observed: 1) The growth and propagation of veins can be affected by the presence of clay seams; 2) Prominent en échelon sets of veins in the HVB are associated with the inception of backthrust ramps; 3) Veins develop along flats and forethrust ramps; 4) Vein sets, excluding joint-parallel veins, are associated with folding (including veins formed in association with flexural slip); 5) Fault-parallel veins, as well as veins that accommodated flexural slip, form in overlapping wedges with lineated surfaces and, in certain cases, occur in association with a set of sub-perpendicular veins.
The alignment of clay minerals along an individual plane (typically parallel to bedding) commonly provides a surface along which slip is more likely to occur. These same clay-lined surfaces can also act as conduits for fluid and as free surfaces where fractures terminate. These characteristics provide conditions for veins to form parallel to clay seams, terminate at clay seams, or only propagate across clay seams when there is a significant amount of opening. The most prominent vein sets visible in outcrops consist of sub-horizontal, en échelon veins that are slightly sigmoidal and range in size from 5 to 30 centimeters thick. Based on their geometry relative to the overall vergence of the fold-thrust belt, these are associated with backthrusts. In some cases the veins link together to form a through-going backthrust, in others the orientation of the enveloping surface is parallel to an incipient backthrust.
Fault-parallel veins are those veins that form parallel and adjacent to the plane of a fault and are lineated on their surfaces. Numerous examples are found in the Hudson Valley Fold-Thrust Belt along detachments, ramps, and bedding-parallel flexural-slip surfaces. They are composed mostly of calcite, although a few examples may contain some quartz. Traditional interpretations of these veins characterized them as thin sheets of slip-parallel calcite fibers, known as slickenfibers or fibrous slip lineations, formed by micro-scale crack-seal deformation. However, this study shows that they are actually blocky, overlapping, grooved, wedge-shaped veins. The wedges grow incrementally along their length in a direction parallel to the slip direction of the fault. The lineations are grooves carved into the surfaces of the wedges by asperities on the overriding wall. Therefore, this study defines a new term “slickenveins” to more accurately describe the fault-parallel veins.
The overall geometry of fold-thrust belts has been compared to the wedge formed by pushing dirt with a bulldozer. The size and shape of this wedge is defined by critical-taper theory. The bulldozer in an actual fold-thrust belt is called the backstop, and it delineates the hinterland-most end of the fold-thrust belt wedge, and it is where deformation related to the fold-thrust belt ceases. Sandboxes with moveable walls or floors have been used as analogs to model fold-thrust belts. Recently, analog models have been analyzed by Particle Image Velocimetry (PIV) techniques in order to track the kinematics of wedge formation, and the location and evolution of specific structures within fold-thrust belts. By using PIV, this project was able to analyze the evolution of the backstop position within the wedge. Results of the model indicated that the size and shape of the backstop changes throughout the formation of the fold-thrust belt. As previous studies have noted, it is important to distinguish between a static and dynamic backstop during wedge evolution. The static backstop remains largely unchanged during deformation. The dynamic backstop exists from the formation of a new forethrust-backthrust conjugate pair until that forethrust branch line reaches the static backstop. Then, a new forethrust-backthrust conjugate pair initiates at the foreland edge of the thickened wedge. At any given time, the foreland boundary of the dynamic backstop can be defined by the boundary separating deforming rock in the foreland from non-deforming rock in the hinterland.